(Note) JRE is a Java runtime environment that contains the Java Virtual Machine (JVM) and Java base class libraries. Is for users who want to run Java programs.

The JDK is a Java development kit, which is used by programmers to write Java programs using the Java language.

JRE: Java Runtime Environment JDK: Java Development Kit The JRE is a Java Runtime Environment that contains the Java Virtual machine (JVM) and Java basic class libraries. Java is the software environment that programs written in the Java language need to run. It is provided to users who want to run Java programs. The JDK, as its name implies, is the Java development kit, which programmers use the Java language to write Java programs. It is provided to programmers. JDK includes JRE, but also contains the compiler javac compilation Java source code, but also contains a lot of Java program debugging and analysis tools: JConsole, JVisualVM and other tools, but also contains the Java program to write required documents and demo examples. If you need to run Java programs, just install the JRE. If you need to write Java programs, you need to install the JDK.

There are many versions of JRE for different operating systems (e.g., Windows, Linux, etc.) and different JRE vendors (IBM,ORACLE, etc.). Java source programs are first compiled by the Javac compiler into binary. Class bytecode files (Java cross-platform means. Class bytecode files are platform-independent), and.class files run on the JVM. The Java interpreter (part of the JVM) interprets it as machine code execution for the platform, so what Java means by cross-platform is that.class files generated on different JVMS on different platforms are the same, and.class files are interpreted by the JVM for the platform to be machine code execution for the platform
Finally, the difference between machine code and bytecode is explained: first, machine code is completely dependent on hardware, and different hardware has different embedded instruction sets, even the same 0, 1 code

In other words, there is no such thing as cross-platform. For example, if you give a CPU of a different model, say 10001101, they may parse different results. As long as you have the JVM installed, it will know the JAVA bytecode ～～～～ as for the underlying machine code, we don’t care. Once the JVM is done, it will translate the bytecode into the machine code that the machine knows